Sammanfattning: Seasonal variation in mood and subjective well-being are common at geographical locations further away from the equator. The 24-h light-dark cycle is the main time cue for synchronizing the human circadian clock to the external day and night.Nowadays, people spend more of their waking day indoors, with less exposure to the natural daylight cycle, relying on artificial lighting which differs to daylight in a number of aspects, including intensity, spectral composition and light exposure pattern.In parallel with the technology development that has been mainly driven by energy-saving reasons, it is important to investigate the non-image-forming effects of different properties of the daily and seasonal light exposure.The overall aim of the thesis was to identify characteristics of the daily light exposure that are important to support physiological and psychological needs of humans. To achieve this objective a number of research questions were posed concerning daily and seasonal light exposure, seasonal variation in physiological processes and psychological parameters, and evaluation of light exposure with respect to non-image-forming effects. The research questions were investigated in a longitudinal research design with measurements conducted each month during the year at a high latitude with large seasonal variation in day lengths.Self-report diaries and instruments for ambulatory- and static measurements were used to examine daily and seasonal light exposure in the working and living environments and for investigating the relationship between different parameters that can be used for evaluating light exposure according to non-image-forming effects of light. Seasonal variation in daily light exposure and regarding the pattern of light exposure was observed. Also, the results indicate a seasonal variation concerning the quality (i.e. spectral composition of the visible radiation) of the exposing light.Two biological markers, melatonin and cortisol, were used for investigating seasonal variation in physiological processes relating to the circadian clock. The results showed higher morning melatonin concentrations and peak level of melatonin during the winter although no seasonal change was observed concerning the phase position of the melatonin rhythm.Seasonal differences in mood and sleep-activity were studied by means of selfreport diaries and questionnaires. Seasonal variations were observed for both parameters. The results showed higher ratings of mood in the summer, particularly 6 in the evening, and a relationship between bedtime and evening light exposure and photoperiod length. Furthermore, longer sleep times was observed in the winter.Appraisal of lighting conditions in the offices during the year was rated by the use of a questionnaire. The results showed some seasonal differences concerning the perceived qualities of the light and some associations between characteristics of the lit environments and positive affect were found.Two methods, static- and ambulatory measurements, were used for recording lighting conditions in the working environments. Taken together, the results showed weak associations between the two methods.Research have demonstrated an increased need for taking non-image-forming effects into consideration when designing working and living environments, especially at geographical locations with large variations in day length where people are exposed to much of the daily light exposure at the workplace. Laboratory research has provided a good understanding of the basic concepts. However, more field research is needed. Also, current research has demonstrated that new methods of measuring and evaluating lighting conditions are needed.